The present invention pertains to the valve art and, more particularly, to means for extending the life of a spool valve.
Spool valves are well known in the hydraulic valve art. Here, a spool, having land portions thereon, rides within the sleeve cavity of the valve body. Fluid is routed through provided passageways in the valve body and accesses the sleeve cavity via metering orifices. These orifices, in cooperation with the lands on the spool, determine flow through the valve.
A particular problem with existing spool valve designs, particularly in the commercial aviation art, is erosion of the valve metering surfaces. Commercial aircraft generally employs phosphate-ester based hydraulic fluids due to their excellent fire resistant properties. These fluids, however, have been found to be highly erosive to metering surfaces.
Research on extending the life of spool valves has taken two approaches: (1) improving the hydraulic fluids, and (2) developing valve designs which erode more slowly. Progress in the former approach has been disappointing, thus the concentration on improved valve designs.
In one prior art valve design, a series of discs is stacked within the ports of the valve thus creating a labyrinth. With a labyrinth, the fluid pressure drop occurs over a relatively long path so that acceleration of the fluid is decreased at the meter surface thus reducing erosion. While tests have confirmed that this porting design extends the useful life of the valve, the labyrinth ports are difficult and, thus, costly to manufacture.